travis: use python3.5 for meson

travis: use python3.5 for meson

Newer Meson versions require python >=3.5. But in Trusty default python3
version is 3.4.x.

Install python3.5 and makes it the default version for Meson using
update-alternatives method.

CC: Jan Vesely <jano.vesely@gmail.com>
CC: Andres Gomez <agomez@igalia.com>
CC: Emil Velikov <emil.l.velikov@gmail.com>
CC: Jon Turney <jon.turney@dronecode.org.uk>
CC: Eric Engestrom <eric.engestrom@intel.com>
CC: Dylan Baker <dylan@pnwbakers.com>
Fixes: 3824c8e7cda97c3bf856 "meson: disable asserts by default on release builds"
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Andres Gomez <agomez@igalia.com>